One of the most familiar and frequently used hand tools is the adjustable wrench, which is a tool for turning various shaped members, such as bolts, nuts, pipe, etc., where great resistance to turning is usually encountered. Although there are number of relatively sophisticated wrenches which are designed for special purposes, one of the most versatile is the simple adjustable crescent wrench, which is available in three or four different sizes. The single most significant advantage of an adjustable wrench over a fixed or single size wrench is that a single adjustable wrench can be used on any bolt or nut from the smallest size possible up to the maximum size to which the wrench can be opened. Thus, one adjustable wrench may take the place of several fixed wrenches and still be able to fit hardware which would be impossible to fit with a fixed wrench.
Although adjustable wrenches can be found in a variety of shapes and design, they are all basically of the same general arrangement. A fixed jaw is mounted on or formed intregrally with an elongate handle, and a movable jaw is mounted by suitable means, such as a groove or trach, for movement toward and away from the fixed jaw. Typically, a knurled driver is rotatably mounted on the wrench in a location where it can be easily turned by an operator, the driver being either internally or externally threaded. The movable jaw has a portion provided with teeth or tooth segments, usually arranged as a rack, which are complimentary to the thread on the driver, the arrangement being such that rotation of the driver by an operator is translated into linear movement of the movable jaw, thus permitting the space between the jaws to be adjusted within the limits of the particular wrench.
A principal disadvantage of wrenches of the type described above is that it is difficult to maintain a given separation of the jaws with any degree of precision due to the fact that the threads on the driver and the teeth or tooth segments on the rack are relatively large and there is considerable free play between the driver and the movable jaw rack. This degree of free play or looseness is necessary to make the wrench easy to adjust, whereas, a more precise fit and closer tolerance on these parts would make the wrench much harder to adjust. As a consequence, an operator is usually constantly adjusting the wrench while using it to turn the same size nut.
Another disadvantage is that the thread pitch on the driver and on the rack is coarse to permit the wrench to be adjusted rapidly since the driver is rotated with just the thumb and forefinger. With a fine thread pitch, it would take so long to make large adjustments to the wrench that the average user would become exasperated, even though a wrench with a fine thread pitch could be more precisely adjusted and might hold a given adjustment tightly enough to remain locked onto a bolt head or a nut.
Still another disadvantage of a conventional adjustable wrench is that the threads on the driver and the teeth or tooth segments on the rack become so rusty, corroded or dirty after extensive use that it is extremely difficult to turn the driver. Often it becomes necessary to actually push the movable jaw while simultaneously rotating the driver in order to adjust the wrench. While it would seem that a few drops of oil might solve this problem, this is not the case because either the driver, the rack or both are exposed and are touched during normal operation of the wrench, with the obvious result that the oil or other lubrication, along with the rust, corrosion or dirt, would get on the user's hands.
Another disadvantage of conventional adjustable wrenches is that they must be made in several different sizes, usually every two inches from four to twelve inches of handle length, depending on the range of the jaw opening and the anticipated torque which the wrench is designed to withstand. This is particularly true of the well known crescent-wrench which has a somewhat limited jaw size range because of the limited amount of movement of the movable jaw.